The invention is situated within the field of monitoring the ageing of parts subject to a rotary movement. It relates in particular to monitoring the rotating elements of a wind turbine. More specifically, the invention relates to a method for monitoring the deformations of a surface of a part capable of undergoing a rotary movement, by means of a measuring device comprising an optical fibre acting as a sensor.
It is known to monitor static civil engineering structures such as a building, tunnel or bridge, by determining their overall deformations, i.e. deformations between at least two relatively distant points of the structure. This is made possible in particular by the installation of an optical fibre stretched between two points of the structure, and by analysis of the variation in a light signal transmitted via the optical fibre. For example, document EP 0 649 000 A1 describes a system for monitoring a building comprising two support plates and an optical fibre fastened to each of the support plates and extending between them to form arcs. Measurement of the amplitude of a light signal passing through the optical fibre makes it possible to determine the variation in the curvature of the arcs, and therefore the relative displacement of the two support plates.
In a machine, some parts, in particular moving parts, are at risk of breakage. However, most of the time it is impossible to equip these parts with optical fibres that are suitably arranged relative to the likely damage, which may be diverse. For example, the deformations of a rotor blade of a wind turbine cannot be monitored by installing sensors on the rotor blade. The same is true for a propeller, a piston, and more generally for any moving part. Furthermore, even in the event that such sensors can be installed on the part to be monitored, the movement of this part presents several difficulties. Firstly, the sensor's measurement can be disturbed by inertial or centrifugal forces or, where applicable, by a gravitational force that varies as a function of the angular position of the part equipped with the sensor.
Secondly, the installation of sensors on a rotating part introduces asymmetry that is likely to accelerate the ageing of the part itself or of another part of the mechanism, for example a mechanical bearing. Finally, the transmission of the measurements to an external device such as a monitoring station, via electrical conductors, is impossible. This problem of transmission of the measurements becomes even more difficult when a large number of sensors is necessary for monitoring the condition of the part.